1. Field of the Invention
The present invention relates to the field of photoelectric solar cell arrays, and to fabrication processes utilizing, for example multijunction solar cells based on III-V semiconductor compounds fabricated into multi-cell modules or subassemblies of such solar cells, and an automated process for mounting and interconnection of such subassemblies on a substrate or panel.
2. Description of the Related Art
Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology not only for use in space but also for terrestrial solar power applications. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 27% under one sun, air mass 0 (AM0), illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. Under high solar concentration (e.g., 500×), commercially available III-V compound semiconductor multijunction solar cells in terrestrial applications (at AM1.5D) have energy efficiencies that exceed 37%. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions.
Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures. The individual solar cells or wafers are then disposed in horizontal arrays, with the individual solar cells connected together in an electrical series circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
In satellite and other space related applications, the size, mass and cost of a space vehicle or satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads become more sophisticated and require more power, both the power-to-weight ratio (measured in watts per kg) and power-to-area ratio (measured in watts per square meter) of a solar cell array or panel becomes increasingly more important, and there is increasing interest in lighter weight, densely packed solar cell arrays having both high efficiency and low mass.
Space applications frequently use high efficiency multijunction III/V compound semiconductor solar cells. Compound semiconductor solar cell wafers are often costly to produce. Thus, the waste that has conventionally been accepted in the art when cutting the rectangular solar cell out of the substantially circular solar cell wafer, can imply considerable cost.
Solar cells are often produced from circular or substantially circular wafers sometimes 100 mm or 150 mm in diameter. Large solar cells (i.e. with, for example, an area from 25 to 60 cm2 representing one-quarter or more of the area of the wafer) are conventionally preferred so as to minimize the costs associated with the assembly of the solar cells onto a support to form a solar cell module. However, the use of large solar cells results in poor wafer utilization, and large solar cells often present issues of defects or variation in the material quality across the surface of the wafer. Also, larger solar cells are fragile and present handling challenges during subsequent fabrication steps that result in breakage of the wafer or solar cells and corresponding lower manufacturing yield. Moreover, large solar cells of predetermined size cannot be easily or efficiently accommodated on panels of arbitrary aspect ratios and configurations which may vary depending upon the “wing” configuration of the satellite or space vehicle. Also, large solar cells are rigid and can sometimes be problematic in terms of meeting requirements for flexibility of the solar cell assembly or solar array panel. Sometimes, flexibility is desired so that the solar cell assembly or the solar array panel can be bent or rolled, for example, so that it is displaceable between a stowed position in which it is wound around a mandrel or similar, and a deployed position extending outward from, for example, a space vehicle so as to permit the solar cells to receive sunlight over a substantial area. Sometimes, large solar cells can be problematic from the perspective of providing a flexible assembly or panel that can be readily bent, wound, etc. without damage to the solar cells and their interconnections.
It is possible to reduce the amount of waste by dividing a circular or substantially circular wafer not into one or two single cells, but into a large number of smaller cells. By dividing a circular or substantially circular wafer into a large amount of relatively small cells, most of the wafer surface can be used to produce solar cells, and the waste is reduced. For example, a solar cell wafer having a diameter of 100 mm or 150 mm and a surface area in the order of 80 cm2 or 180 cm2 can be used to produce a large amount of small solar cells, such as square or rectangular solar cells, each having a surface area of less than 5 cm2, or in some embodiments less than 1 cm2, less than 0.1 cm2, less than 0.05 cm2, or less than 0.01 cm2. For example, substantially rectangular—such as square—solar cells can be obtained in which the sides are less than 10, 5, 3, 2, 1 or even 0.5 mm long. Thereby, the amount of waste of wafer material can be substantially reduced, and at the same time high utilization of the wafer surface can be obtained. Also, when dividing a solar cell wafer into a relatively large number of solar cells, solar cells obtained from a more or less defective region of the wafer can be discarded, or “binned” as lower performance solar cells, that is, not used for the manufacture of the solar cell assemblies. Thus, a relatively high quality of the solar cell assemblies in terms of performance of the solar cells can be achieved, while the amount of waste is kept relatively low.
However, the use of a large number of relatively small solar cell involves the drawback that for a given effective surface area of the final solar cell assembly or solar array panel, there is an increased number of interconnections between solar cells, in a parallel and/or in series, which may render the process of manufacturing the solar cell assembly or the panel more complex and/or expensive, and which may also render the entire circuit less reliable, due to the risk for low reliability, low yield, or other manufacturing difficulties or errors due to defective or less-than-ideal interconnections between individual solar cells.